The present invention relates to communications systems and methods, and more particularly, to spread spectrum communications systems and methods.
Wireless communications systems are commonly employed to provide voice and data communications to multiple recipients. Several different types of access techniques are conventionally used to provide such wireless services. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity. More recently developed digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or TIA/EIA-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using “spread spectrum” techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code, i.e., a code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional CDMA systems typically code information to be transmitted and map the coded bits in pairs to QPSK symbols. The QPSK symbols are then spread by a factor of M to a chiprate and bandwidth determined for the CDMA system using one of a set of M, M-symbol orthogonal spreading codes.
In such conventional systems, an increase in the coded bitrate by a factor of two could be accomplished by reduction of M by a factor of two to obtain half the number of orthogonal codes of half the length and twice the information rate. This so-called variable-rate, orthogonal coding is specified for the 3rd generation cellular system known as UMPTS or IMT2000. The number of higher bitrate accesses may be limited by the number of available orthogonal codes, even if not limited by the available transmission power or signal-to-noise ratio.
In other conventional systems, higher data rates can be obtained for a given user by allocating him more than one of the available codes, the so-called “multi-code” solution. However, this typically reduces the number of codes available to allocate to other users, and may limit the number of simultaneous users served by the system.
FIG. 1 illustrates a conventional use of variable-rate Walsh-Hadamard codes orthogonal codes for a CDMA system. The structure of these Walsh-Hadamard codes begins with a pair of elementary two-bit codes 11 and 10. These codes are orthogonal because half the bits agree and half the bits differ, giving a cross correlation of zero.
The two, 2-bit codes may be expanded into four mutually orthogonal 4-bit codes by taking each 2-bit code in turn and expanding it to two, 4-bit codes, the first of which comprises the 2-bit code repeated, and the second comprises the two bit code repeated with the repeat inverted (complemented), thus assuring that half the bits agree and half the bits disagree between the two new 4-bit codes. Likewise, each 4-bit code can be expanded into two 8-bit codes, by repeating the four bit code twice for one 8-bit code, the repeat being inverted for a second 8-bit code, and the process may continue indefinitely to obtain a set of any number of orthogonal codes.
Binary traffic information is transmitted by transmitting an assigned code to represent a “1” or its inverse to represent a “0.” One information bit is thus conveyed per 16 “chips” in the case of a 16-bit orthogonal code being used, that is the information rate is 1/16th the chip rate. The penalty of using longer codes, of which a greater number are available, is thus a lower data rate.
If it is desired to construct orthogonal codes of length other than a power of two, Fourier sequences may alternatively be used as described in U.S. patent application Ser. No. 08/898,392, entitled “COMMUNICATION SYSTEM AND METHOD WITH ORTHOGONAL BLOCK CODING”, filed Jul. 22,1997, in U.S. patent application Ser. No. 09/340,907, entitled “MULTI-CARRIER ORTHOGONAL CODING”, filed Jun. 28, 1999, and in U.S. patent application Ser. No. 09/082,722, entitled “PARTIALLY BLOCK-INTERLEAVED CDMA CODING AND DECODING”, filed May 21,1998.
Fourier sequences of a composite length such as 2×3×5 may be constructed to allow variable rate orthogonal codes for changing the data rate in successive steps of 5, 3 and 2, i.e. in steps equal to the factors of the composite length, but restricted to the order in which the factors are employed to construct the code. For example, if the code is constructed with Fourier sequences of length 5×2×3, then the successive steps in which the data rate may be changed are factors of 3 then 2 then 5, instead of 5 then 3 then 2 as with the first example.
Future mobile communications services will probably provide for a variety of different types of traffic, including high-penetration short message service, which is probably will be the lowest bitrate service and probably will use a long code, such as 1024 chips, and digitized voice, which is likely to be the next lowest bitrate service and probably will use a shorter code, for example, 256 or 128 chips. The highest bitrate service could be mobile Internet or a “videophone” service, which might use only 16 or even 4-chip codes,
When a shorter code, such as 4 chips, is used for a high bitrate service, the two 8-bit codes which could have been derived from it by the expansion shown in FIG. 1 will typically not be used, in order to maintain orthogonality of transmission. In effect, the use of one 4-bit code can thus consumes the code space of two 8-bit codes or four 16-bit codes, and so forth.
A different 4-bit code that is not used for a high bitrate service can, however, be expanded to two 8-bit codes or beyond, to provide lower bitrate services. Because the latter are derived by repeating a 4-bit code that is orthogonal to the 4-bit code used for the high bitrate service, all of the lower bitrate signals are orthogonal to the high bitrate signal as well as to each other. Thus the technique of variable-rate orthogonal coding described above with reference to FIG. 1 can allow a mixture of traffic types subject to each instance of a particular traffic bitrate denying two instances of traffic at half the bitrate, or four instances of traffic at ¼ the bitrate, and so on.
Thus, with many instances of high bitrate traffic, there may be a large loss in the number of codes available for low bitrate traffic, such as voice. Moreover, it may be commercially disadvantageous for an operator of wireless communications services to charge for high bitrate services at a level commensurate with the number of voice traffic signals thereby displaced. For example, a 1 MB/S Internet connection might displace 64, 16 kilobit voice signals that normally yield a revenue of 10¢/minute each; however, it would may be commercially infeasible for the operator to charge $6.40/minute for the Internet service that displaces these 64 voice signals.
At least one cellular system, known as GSM/EDGE, has recognized this economic dilemma and has proposed to use binary GMSK modulation for voice and 8-phase modulation for data, thus tripling the bitrate for data in comparison to voice. In this manner, the resource usage penalty in providing high bitrate services compared to voice services may be mitigated by a factor of at least 3. A potential disadvantage of using 8-psk modulation, however, is that an increase in transmitted power may be required to maintain a given transmission error rate, or an increase in transmission error rate may need to be tolerated. Higher layers of the packet protocols normally used for data transmission can tolerate and compensate for increased error rate by smart acknowledgement and retransmission strategies. However, these error mitigation techniques may not be desirable for voice services because of a desire to provide real-time transmission. Consequently, the cost of data services may be reduced by accepting variable transmission delay due to errors, which may be tolerable in many Internet applications, for example.
FIG. 2 illustrates a conventional CDMA system that uses QPSK modulation, which is a four-phase modulation conveying two bits per information symbol. In FIG. 2, information bits enter a turbo-encoder 10 that performs error correction coding and outputs coded bits. The number of coded bits is greater than the number of information bits by a factor in the region of 2-4, reflecting the rate of the code. The coded bits are separated into two streams by, for example, directing even-numbered bits to one output and odd-numbered bits to another output using switch 1. Each of the even or odd bits is then expanded by a CDMA spreading factor to provide a greater number of chips per second, using the same one of a set of orthogonal spreading codes for both the even and the odd bits. The even and odd bits and the corresponding even and odd chips produced by spreading 13 using assigned code 12 are destined to be transmitted using the cosine and sine or “I and Q” channels of a complex radio signal modulation, and so may be called “I-bits, Q-bits” and “I-chips, Q-chips” respectively. A pair of bits comprising an I-bit and a Q-bit, if the bit periods are synchronized, comprises a QPSK symbol, and a pair of I,Q chips comprises a QPSK symbol at the CDMA spread rate.
The I-chips are filtered before modulating a cosine radio carrier signal, and the Q-chips are filtered before modulating a sine carrier signal, in a quadrature modulator 16. Prior to modulation in modulator 16, however, the I and Q chip streams are scrambled by rotating the phase of the QPSK symbol or complex number it represents through either 0,90,180 or 270 degrees by complex-multiplication with a pseudorandom QPSK symbol stream from an I,Q code generator 15. This scrambling code is typically common to all transmitters in the CDMA system, so it typically does not destroy the mutual orthogonality between different assigned codes 12 used by different transmitters. The transmitter of FIG. 2, can use different length assigned codes (12) depending on the desired traffic bitrate.
A QPSK symbol steam comprises an I-bit of chip stream and a Q-bit or chip stream to transmit twice the data rate. This typically can be done with little or no penalty over a BPSK scheme because the I and the Q channels use respective cosine and sinewave carriers that are at right-angles to each other and do not interact. This is the basic (i.e. lowest traffic rate) modulation scheme typically used in a conventional CDMA system such as that illustrated in FIG. 2.
A typical receiver for signals transmitted by an apparatus such as that in FIG. 2 includes corresponding receiver processing blocks in reverse order. A quadrature demodulator receives the signal and reproduces I and Q sample streams. The I and Q sample streams are complex multiplied by the conjugate of the common code generator code to undo the scrambling phase rotations applied at the transmitter. The phase-unscrambled I and Q streams are then correlated with the assigned orthogonal code by multiplying successive samples by the bits of the code and summing over the code length. The resulting I-sums and Q-sums, now at the despread rate equal to the original coded bit rate from turbo-encoder 10 are processed by a turbo decoder to reconstruct the original information. Other refinements in CDMA receivers can include RAKE combining of multipath rays and other conventional techniques.